The present invention relates generally to hardness testing of a sample, and is more particularly directed to an instrument for measuring and indicating the hardness of a sample of plastic foam, both of the relatively rigid type such as polystyrene as well as more resilient plastic foam material. It may be noted that, for any given plastic foam material, the density of the material is a function of its hardness, and as a consequence, the present instrument may be calibrated to yield a reading of density.
In the preferred form of the invention hereinafter described and illustrated, there is provided a frame which may be easily grasped by the user, the frame having mounted thereon a thin elongated rigid indentor, preferably of rigid metal, provided with a downwardly convex marking edge. The exact contour of the marking edge may be selected to be particularly adapted for a specific material of a sample to be measured; in the present embodiment of the invention, the contour of the marking edge is that of a portion of a circle which may have a radius, for example, of 2 or 3 inches. The frame includes a lower plate having a contour which is matingly congruent to the contour of a sample to be tested, and is provided with a thin elongated slot for loosely receiving the indentor therein. In the present illustrative form of the invention, the contour of the lower plate is planar for simplicity of description and illustration, but it will be understood that the plate contour could be of other shapes, to be matingly congruent with a sample. The plate serves as a means for limiting movement of the frame in the direction of the sample so that the frame proper will not penetrate the sample.
Calibration means are provided for selectively adjusting the amount of compression exerted by the resilient means biasing the indentor downwardly relative to the frame. Such means in the preferred form of the invention include a calibration bar against which the resilient biasing means abut for urging the marking edge into the sample, and means for selectively adjusting the position of the calibration bar relative to the frame proper.
In use, the user first applies a coloring material or ink to the indentor marking edge, most conveniently by simply rolling that edge on a conventional stamp inking pad. The instrument is then applied to the sample by contacting the marking edge with the surface of the sample, and manually pressing the frame toward the sample and into abutting contact with portions of the sample immediately adjacent to that contacted by the marking edge. The instrument is then removed from the sample, and the length of the indentation is measured, such length being clearly shown by the transfer to the sample of the ink or other coloring material on the marking edge. A calibrated scale for a particular composition of sample being measured is conveniently attached to the frame for making the linear measurement just referred to.
Accordingly, it is the principal object of the present invention to provide a novel instrument for measuring and indicating the hardness of a sample of material such as a plastic foam material. Additional objects and purposes of the invention are to provide, in such an instrument, a thin elongated indentor having a convex marking edge, the indentor being resiliently mounted in a manually grasped frame; to provide in such an instrument a plate for contacting the sample, the plate being provided with a slot for loosely receiving therein the indentor; to provide, in such an instrument, calibration means including a calibration bar serving as the upper seat of resilient means biasing the indentor into the sample, the calibration bar being selectively positionable relative to the frame; and for other and similar objects and purposes as will become clear from a reading of the following description of a preferred embodiment of the invention, taken in connection with the accompanying drawing.